Surface modification aims to tailor the surface characteristics of a material for a specific application without detrimentally affecting the bulk properties. At present a range of methods is used to effect surface modifications on a wide range of materials, including biomedical devices and biomaterials, wood, textiles, leather, metals, glass, ceramics, paper and plastics.
Such finishes may for instance include wettability, water-repellent and waterproofing finishes; coloration, lacquering, and abrasion protection finishes; chemical softening, easy-care, antistatic and soil-release finishes; flame-retarding finishes; and anti-microbial, rotproofing and hygiene finishes. The finish itself constitutes a chemical substance bonded to the surface by mechanical or chemical interaction.
The application of these finishes requires a specific application process and must be tailored to the material and the desirable properties. In fact, as each material differs in its surface properties the application of a finish layer thereon may require specific adaptations of the process. For instance in the field of textile finishes, such parameters as fiber nature (100% natural, synthetic or blends thereof) and inherent absorbent properties as well as weave and construction of a textile fabric, largely determine the possibility of subjecting the material to a wet finish process. In addition, the properties of the finish are very much determined by the process used. For instance when a smooth finish is to be obtained on a rough surface, a wet process that deposits large amounts of finish material is preferred.
In the field of regatta sailcloths, important improvements can be expected from surface finishes. Modern sailing regattas are high-tech events comparable to F1 car racing. Constant development of new sailcloths, weaves, yarns, laminates, foils and reinforcement fibres is extremely important for regatta yachting. The spin-off benefits of these high tech developments results in improved sailcloths for the coastal and cruising sailor.
The majority of the sails are woven cloths, often based on polyester fibres such as a polyethylene terephthalate (PET), also known as Dacron®, which provides for a durable, easy to handle and reasonably priced product. Over the years these relatively traditional weaves have been improved with respect stretch and UV resistance, durability, and ease of handling and maintenance resulting in the production of sails based on ultra high molecular weight polyethylene (UHMWPE, e.g. Spectra® and Dyneema®), liquid crystal polymer (LCP, e.g. Vectran®), polyethylene naphthalate (PEN, e.g. Pentex®), or aramid (e.g. Kevlar® and Twaron®) fibers.
In search for lighter materials, laminate sailcloths have been developed that consist of 3-5 alternating layers of a woven material, for instance in the form of a taffeta (silk weave), scrim (loose mesh) or inlay (strands) as the primary load carrier and for abrasion resistance, and a film material such as PET (e.g. Mylar®) or PEN for holding the fibres in place and providing stretch resistance. These layers are glued together to provide the laminate. Laminate sailcloths are stronger and more stretch resistant and therefore particularly useful for larger sail areas common for the larger yachts. Their low weight also makes these sails easier to handle and improves sailing efficiency, however, at the expense of increased costs and reduced durability, since these laminates are prone to de-lamination and mildew. In addition, improved water-repellency is required.
Water introduced between the sheets or the seams of the sailcloth is a serious cause of fungal growth. An increase in the water-repellency of the sailcloth reduces the infiltration of water in the sailcloth. In addition, because weight is very important for sailcloths, water-repellency prevents accumulation of water and dirt (anti-staining), and provides for stable lightweight characteristics.
It is well known that the hydrophobicity of a surface determines its water-repellency. One existing method for providing hydrophobicity uses a wet finish process which involves the application of a liquid coating solution to the material surface and an intensive post-application treatment to activate the hydrophobic properties of the coating. These wet coatings are not very durable as they are insufficiently permanent. Also, large amounts of coating solution are required making the process costly. Most importantly these methods result in a significant increase in the weight of the material.
An example of such a wet finishing process is described in GB114782. GB114782 describes the use of fluorine-silicon adducts and the impregnation of a textile fabric by spraying or dipping with a solution of the fluorine-silicon. This is an example of a wet finish process that does not result in a monomolecular layer. The disadvantage of that method is that the fabric becomes too heavy.
Another example of such a wet finishing process is described in EP0588242. EP0588242 describes the covalent binding of chlorosilane based chemical adsorbents to a material surface wherein a in the form of monomolecular film bonded to materials via a chlorosilane layer in order to render these material water and oil repellent. The process of EP0588242 comprises contacting the material with a chlorosilane solution to adsorb the chlorosilane to the material, removing the unbound chlorosilane and reacting the unreacted chlorosilyl groups of said adsorbed chlorosilane with water to form a chemically adsorbed monomolecular film. Subsequently, a chlorosilane-based chemical adsorbent having fluorocarbon groups is chemically adsorbed to this film, thus forming a chemically adsorbed monomolecular film having water- and oil-repelling properties. This method comprises several steps and is not a one-step process. Furthermore, the alkylsilane is not bonded directly to the surface of the fabric.
Another existing method uses a gas phase process involving the deposition of gaseous precursors under the influence of a plasma.
Gas plasma treatment has the advantage that very thin layers can be deposited. However, the problem with such methods is that they are very expensive as dedicated equipment is needed for applying the coating. In addition, these methods are very difficult to perform on the scale needed for treating the large surface areas of sails as the processes must be carried out at reduced pressure in a treatment chamber housing the plasma source.
Thus, the problem with current methods for hydrophobing a material surface, and in particular a material having a large surface area such as a sailcloth, is that these methods result in a significant increase in weight and that the coating is insufficiently durable, or that they are not economical.
The aim therefore is to provide an economic method by which a hydrophobic functionality can be added to the cloth without significantly increasing its weight, and without compromising durability and wear resistance.
GB1023897 discloses the use of a two-step method for rendering fibrous material water-repellent comprising a first step of using an organic ester of titanium or an alkyl tin carboxylate as a catalyst followed by a second step involving the treatment with a vaporous fluorohydrocarbon alkoxysilane having an alkoxygroup as hydrolysable group. The drawback of this process is that it is very uneconomical to perform two separate treatment steps on a large areas of material such as a sailcloth.